Is there an expression for a person who eats very little, doesn't like eating, avoids it?
I don't mean the medical condition of anorexia, I mean a common preference, like kids who need a lot of convincing to eat their lunch or adults who can run a whole day on a single donut and realize this is wrong but simply don't care and won't eat a solid meal unless you shout at them.
Very informal or even slang words are okay.
Someone who eats like a bird is someone who "eats in small amounts rather than in a single full meal."
What you mention does sound like anorexia, (which on its own just means "not eating much") but not anorexia nervosa for which anorexia is so often used as a shorthand that the word is a fraught one to use in any other context. A word that requires you to explain how you mean it, is a word that's no longer pulling its weight.
You could have inedic as an adjective formed from inedia, but inedia is so obscure as to be pretty useless too.
Saying someone "lives on air" is sometimes used as a half-joking colloquialism, and sometimes as an serious claim about inedic yogis. That half-joking use might match your use.
Phobialist describes Sitophobia (or Sitiophobia/Cibophobia) as the fear of food or eating.
Google returns 11,600,000 results for Food Aversion, 14,600,000 for poor eater and 3,020,000 results for picky eater.
The closest thing to your request would I think be "abstemious." This is an adjective (an abstemious person). It can refer to someone who eats and/or drinks in moderation.
Since you specify something short of pathological eating, moderation seems to fit the bill.
I've seen the term light eater used many times. As for an adjective-based description, try perhaps bird appetite.
To complement both Jon Hanna and Fumble Fingers' answers I'd like to add the following:
Fussy eater; fussy eating or more rarely a fussy food eater
is usually a child or a toddler who has a very limited diet by choice, and will refuse to eat a long list of foods based on texture and/or aroma, e.g. all vegetables and fruit. In more severe cases this refusal will include any new food that is introduced which may lead to developing a phobia called neophobia.
An adult who simply forgets to eat is someone who manages to go through an entire day without eating any solids but will often drink copious amounts of sweet coffee, tea or high energy drinks in order to get through the day and make up for the lost calories.
In TV Tropes, Forgets To Eat is described light-heartedly as someone:
... who's notorious for ignoring their body's need for nutrition and has to be reminded to eat. [...] as workaholics and others will become so engaged in something that they do forget to eat. Isaac Newton was famous for being so obsessive-compulsive about his work that he would work hours and hours while neglecting to bathe or eat. Because this is common among obsessed gamers, Anti Poopsocking had to be put in place in many games. Science further backs this up, as the nervous system has the digestive system slow and close up if a person is under stress. Nobody wants to stop for a bathroom break or snack when being chased by a bear.
